The present invention relates to processes for the vacuum treatment of workpieces as well as to corresponding process equipment.
A group of several workpieces is subsequently referred to as "batch".
The simultaneous vacuum treatment of entire batches has been known for a long time. The traditional procedure was to load a vacuum chamber with a batch, to pump down the vacuum chamber and to treat the batch inside the vacuum chamber. After the treatment process the vacuum chamber was vented, opened and the treated batch removed.
This procedure is limited to processes in which a batch treatment can be performed consecutively, usually with intermediate phases in normal atmosphere. This means that such a batch treatment is not feasible for workpieces that require a series of vacuum treatment steps, all or most of which must fulfill demanding cleanliness requirements. For this type of critical process sequences a different treatment technique has been developed in which the workpieces are loaded through a vacuum lock and, without leaving the latter, are subjected to a series of treatment process steps such as coating processes in all known vacuum engineering versions including CVD, LPCVD, PECVD, PVD, as well as etching processes, cleaning processes, heating or cooling processes.
From U.S. Pat. No. 5,344,542, for example, a known solution is to submit workpieces from a load lock hopper station via a centrally arranged vacuum transport chamber to several connected treatment stations for which purpose a transport robot is used in the transport chamber. To minimize cross-contamination between the individual treatment steps in the treatment stations through the transport chamber, the treatment stations can be vacuum isolated relative to the transport chamber by means of valves. The workpieces are transported individually to the desired process stations via the transport chamber.
If different types of treatment stations that satisfy specific requirements are flanged to such a transport chamber this also results in different requirements with respect to the time sequence based on which the individual workpieces must travel through the individual treatment stations, and also with respect to the treatment time to which the individual workpieces are subjected at the corresponding treatment stations. To achieve flexibility in subjecting the individual workpieces to the intended treatment, U.S. Pat. No. 5,344,542 also proposes to make the robot arrangement interact with a process controller at which the required time sequence based on which the treatment stations are served by the robot arrangement can be freely defined.
Also known from U.S. Pat. No. 5,019,233 is the application of the single workpiece treatment principle to highly complex and critical workpiece treatments under vacuum condition, and in particular to treatment steps that are highly sensitive to contamination as discussed in connection with U.S. Pat. No. 5,344,542. It is acknowledged, however, that the workpieces after they have been loaded into an input lock, must first be conditioned, that is, degassed in such a way that subsequently they do no introduce absorbed external gasses as contamination into the highly critical processes. If according to U.S. Pat. No. 5,019,233 this necessary conditioning of the workpieces is performed when they are loaded through the vacuum lock, relatively long conditioning times are required. The vacuum lock becomes the slowest element of the process sequence and significantly limits the throughput of a corresponding treatment plant.
There the problem is solved in such a way that two load locks are operated in parallel. A workpiece batch is transported into the first load lock where it is conditioned and subsequently transferred into a second vacuum lock. From the latter one workpiece at a time is distributed via the central transport chamber to the appropriate treatment stations, again in a user selectable sequence, while already the next batch is loaded and conditioned in the first vacuum lock. For sequencing the highly delicate process steps in the treatment stations, the aforementioned single workpiece treatment process is still followed. Only for loading through the vacuum lock and for conditioning as well as for unloading the workpieces are grouped into batches.
A similar procedure is followed according to EP 0 608 620. Basically also there the workpieces are to be subjected to a complex sequence of individual, critical vacuum treatment steps. Also in this case the aforementioned concept of loading and unloading the workpieces in batch mode through the vacuum lock and subjecting the workpieces individually to the various treatment steps is still maintained.
According to that patent application thin glass substrates are to be treated. An acute breakage problem is said to exist if such substrates are exposed to abrupt temperature changes. However, if such glass substrates are to be individually heated and cooled gradually in single workpiece treatment mode, the throughput of the overall system is significantly impaired as already mentioned in U.S. Pat. No. 5,019,233. As in the case of U.S. Pat. No. 5,019,233 also here the slowest steps of the process sequence, that is, gradual heating and gradual cooling are still performed in batch mode. Besides, these are process steps that are rather uncritical. Also according to this application a batch of workpieces is loaded through the vacuum lock and the loaded batch is then slowly heated as a batch in a heating station. Subsequently, however, the workpieces are transported individually to the various treatment stations and then collected as a batch in the exit chamber where they are gradually cooled before they are unloaded to atmosphere.
As can be seen, the concept of treating each workpiece requiring a complex sequence of process steps individually, that is, one workpiece at a time, was systematically followed. One of the major reasons for this was also the prevailing opinion that with a complex sequence of process steps and relatively costly workpieces to be treated, only individual workpiece treatment was suitable for achieving adequate control over the processes and for controlling, monitoring and reproducing the process sequence, and in addition to limit the damage to a few workpieces in the event of a process fault.
As far as is known DE-OS 44 12 902 was the first application which proposed that also in such highly delicate workpiece treatment processes the workpieces should not only be loaded and unloaded through vacuum locks in batch mode, but also be submitted as a batch to a central transport chamber, from where they are submitted to and handled as batches in the various treatment stations. For further increasing the throughput, several identical or identically operated treatment stations are operated in parallel.